As others have said.
Might I be able to ask how you are currently implementing Google App Engine as the CDN on your forum? I assume you're syncing files up there?
I've got a website running ALL static content, except jQuery, from Google App Engine. Suffice to say it's fine. However, simple template edits are sufficient to satisfy the craving for putting often used content to the CDN.
Additionally, it will help to set ETags and Expires. Many of us dive into the CDN without looking first at what can be done on the server-site side. For example, just set up a cookie-less domain and serve content from that. Not too difficult
